1. Field of the Invention
This invention relates to improvements in the sweetening of natural gas and more particularly to a method and apparatus for sweetening natural gas at the well head or a collection point in the field.
2. Brief Description of the Prior Art
The production of natural gas often requires the separation or removal of various contaminants from the gas before it can be sent on for use. Natural gas often includes a substantial amount of entrained water and vaporized liquid hydrocarbons, usually the more volatile ones. Consequently, the gas is subjected to treatment for separation of these components.
Natural gas may also contain gaseous impurities such as carbon dioxide and hydrogen sulfide which are acids in aqueous solution and thus corrosive. Hydrogen sulfide-containing gas is also highly toxic and malodorous and is referred to as "sour" gas. In fact, hydrogen sulfide is more toxic than HCN and presents the problem that it is highly malodorous at extremely low concentrations and tends to anesthetize the olefactory nerves with the result that a toxic exposure may not be recognized until it is too late. The removal or neutralization of hydrogen sulfide is therefore a matter of necessity from a safety standpoint.
The removal of carbon dioxide is not always required but can usually be removed by the other processes used to remove hydrogen sulfide. In many processes of treatment, the chemicals used for sweetening react with both carbon dioxide and hydrogen sulfide and therefore the total amount of these impurities is used in calculating the amount of treating chemicals needed.
In most procedures, the natural gas is first treated to remove water vapor and to separate condensable hydrocarbons or "condensate". The partial expansion of the gas through a choke to a lower pressure is effective to cool the gas sufficiently to remove both water and volatile hydrocarbons by condensing them from the gas stream. Often, there is a material added, such as ethylene glycol which will absorb or hydrate with the water to condense more readily from the gas stream. The expansion through the choke and consequent cooling is usually sufficient to condense the volatile liquid hydrocarbons which are recovered for use as solvents or fuel, i.e. casing head gasoline.
The major process for removal of acid constituents from natural gas is one using an alkanolamine, such as monoethanolamine (MEA), diethanolamine (DEA), and/or triethanolamine (TEA). Treatment with alkanolamines involves circulating natural gas upward through a treatment tower to contact the alkanolamines. The acid gases react with the alkanolamines to form either a hydrosulfite or a carbonate of an alkanolamine. The alkanolamines admixed with the reaction products are conducted to a stripping still where the alkanolamines are removed and returned to the treatment column. The reaction products are then conducted to a reactor where they are heated sufficiently to reverse the process and regenerate the alkanolamines and release the acid gases which may be flared to convert hydrogen sulfide to sulfur dioxide, or further reacted to a form for solid disposal, or sent to a sulfur manufacturing plant.
There are several variations on the alkanolamine desulfurization process in use. One such process is the Shell Sulfinol process (licensed by Shell) which utilizes a mixed solvent. The Sulfinol solvent is an admixture of sulfolane, water and di-isopropanolamine (DIPA). Another process of this type utilizes a mixture of alkanolamines with ethylene glycol and water. This process combines the removal of water vapor, carbon dioxide and hydrogen sulfide.
Another process for removal of hydrogen sulfide, uses a solid-gas chemical reaction. An iron sponge, consisting of a hydrated iron oxide on an inert support, is treated with the sour gas where the iron is converted to the sulfide. The iron sulfide can be reoxidized to the oxide with release of elemental sulfur.
Some physical processes are used for removal of carbon dioxide and hydrogen sulfide. Molecular sieves, i.e. zeolites and other materials having a pore size of molecular dimensions, which are specific in pore size for removal of carbon dioxide and hydrogen sulfide are used in the form of a bed through which the sour gas is passed. The bed is periodically regenerated by stripping with an inert gas. This process has the disadvantage present in most desulfurizing processes in that the separated hydrogen sulfide or sulfur dioxide must be disposed of in the field.
All of the above desulfurization process have the disadvantage that reaction vessels, strippers, stills, separators and the like must be provided, which have a high capital cost. Also, these processes have the disadvantage that the current laws dealing with air pollution make it difficult to dispose of the separated hydrogen sulfide or sulfur dioxide under field conditions.
The present invention involves the use of inexpensive equipment and reagents for sweetening which avoid the problem of disposal of separated hydrogen sulfide or sulfur dioxide under field conditions.